1. Field of the Invention
The present invention generally relates to the production of high current electron beams and, more particularly, to the production of electron beams of uniform intensity over a large area and a large beam divergence angle particularly applicable to electron beam projection systems and lithography tools.
2. Description of the Prior Art
Numerous industries, especially semiconductor integrated circuit manufacturing, rely on lithographic processes in which a pattern of material is deposited or removed such as etching a pattern into a substrate or a blanket layer of material. Lithographic processes are also used to make masks which may then be used in other lithographic processes. Generally, a layer of resist is applied to a surface and a selective exposure made of areas of the resist layer. The resist is then developed to form a mask by removing either exposed or unexposed areas of the resist (depending on whether the resist is a positive or negative resist) and a material deposited or removed such as by etching, implantation, chemical vapor deposition (CVD) or the like, possibly using a plasma, in a pattern corresponding to the mask.
To produce very fine features (e.g. fine pitch, small feature size and the like) very high resolution is required. Resolution is limited by the wavelength of the radiation used to make the exposure as well as other physical effects presented by the exposure medium. Electron beams have been used as an alternative to radiation to produce exposures at finer resolution than can be accomplished using even very short wavelength (e.g. ultra-violet) light. Extreme ultra-violet (EUV) radiation and X-rays are being investigated but present additional problems.
Electron beam exposure is also convenient for complex patterns since an electron beam can be rapidly and accurately deflected by electrical and/or magnetic fields to serially expose selected areas of the resist such as in direct writing (known as probe-forming systems) or step-and-repeat processes using a mask for shaping the electron beam. These latter processes and apparatus for performing them are referred to as electron beam (or e-beam) projection processes and tools.
Electron beam projection systems which project a potentially complex pattern have much greater theoretical throughput than systems employing spot exposures because the former can produce a complex pattern with a single exposure (generally with a relatively large deflection step between exposures) while the latter is constrained to developing a desired pattern by deflecting the e-beam for serial exposure of all parts of each pattern exposed. At the same time and for a given sensitivity of resist, any realization of an increase in throughput requires an increase in beam current in view of the greater area exposed in e-beam projection systems.
However, some practical limitations on resolution are also characteristic of electron beams. Suitable resists for electron beam exposure require a significant electron flux (e.g. the number of electrons) for exposure. Therefore, throughput of an electron beam (hereinafter sometimes "e-beam") tool is limited by the beam current which can be developed. At the same time, the charge carried by each electron or ion causes a repulsion force between the like-charged particles (generally referred to as Coulomb interactions) which increases with proximity between particles. Accordingly, high density of electron population in the electron beam causes aberrations in the nature of blurring or defocussing in the beam image because of the interactions between the electrons. Therefore, there is a trade-off between resolution/aberrations and maximum beam current and throughput.
At the present time, there are three principal approaches to increasing the useable beam current while containing electron interaction aberrations to a significant degree. Two of these approaches effectively rely on reduction of the average beam current density. The first approach involves the projection of relatively large sub-fields to maintain throughput at lower current density and, if the sub-field is sufficiently large, increased total beam current can be employed without severe detrimental effects of high current density. Further, for reliable exposure over a pattern, the intensity of electron illumination across the subfield which is imaged must be highly uniform, generally within about 1% across the reticle. The second is to use a large numerical aperture which corresponds to a large beam semi-angle at the target (e.g. the average cross-section of the beam is large and sharply converged only shortly before the target through a large angle to the beam axis).
A third approach to the trade-off which allows increase of resolution at a given throughput is to increase beam energy (e.g. a high accelerating potential for the beam). Geometric aberrations (with the exception of chromatic aberrations) are unaffected by beam energy while the trajectory displacement (TD) aberration due to Coulomb interactions and chromatic aberrations decrease with increased beam energy. As the approaches discussed above reduce electron proximity by increasing the beam cross-sectional area at a given current, increased electron energy decreases the time required for an electron to traverse the beam length and allows less time over which the Coulomb interactions can develop electron displacements and consequent aberrations. This can also be conceptualized as a decrease in electron density in the axial direction of the beam.
Large sub-field sizes, large beam semi-angles and high beam energy put stringent demands on the electron source in an electron beam projection system. These demands can best be understood in terms of required source emittance. Emittance is a fundamental property of an electron optical system and is defined as the product of the diameter of the electron emitting portion of the cathode and the half-width of the angular distribution of the emitted electrons. Convenient units for emittance are millimeter-milliradians. Emittance is important because it is conserved throughout the e-beam apparatus in the sense that it cannot be increased within the optical system but, of course, may be reduced by apertures, diaphragms and the like which intercept the fringes or larger outer regions of the beam and thus reduce beam diameter.
Since the optical system cannot increase beam emittance, it follows that the electron source must provide the necessary emittance. Moreover, since the beam semi-angle is proportional to (Vac).sup.1/2, all the approaches discussed above for improving throughput of an e-beam projection system at a given resolution (namely, large sub-field size, large beam semi-angle and high beam energy) demand increased source emittance. The result is that for electron beam projection lithography, an emittance of 2-4 mm-mrad at 100 kV accelerating voltage is needed. This emittance is about one hundred times larger than that of conventional probe-forming e-beam systems.
The only known approach to obtaining such a required emittance is through the use of a cathode with an emission area of diameter one hundred times larger than a conventional triode gun cathode. Cathodes in this size range are known in other applications (e.g. electron beam welders, sources in high-energy particle accelerators and klystrons). However, uniformity of electron emission is not of importance in any of these applications. In sharp contrast, for an electron beam projection system of tool, uniformity is of primary importance.
To obtain high uniformity of emission, assuming that beam intensity uniformity is preserved by a distortion-free electron optical system, a cathode operating point having a particular cathode temperature, emission current and extraction field strength must be achieved in accordance with the chosen emission current and extraction field strength such that cathode emission is determined only by cathode temperature and cathode material work function. If so, since the cathode material and its work function can be controlled, uniformity of emission is principally a function of the uniformity of cathode temperature which can be achieved.
Direct resistance heating of the cathode is preferred for sub-millimeter cathodes such as might be found in electron microscopes. However, for larger cathodes, direct resistance heating is impractical because of the large currents which would be required. Accordingly, indirect heating by electron bombardment is traditionally used for cathodes larger than a few millimeters in diameter.
One known configuration for indirectly heated cathodes is in the form of a rod with a directly heated helical filament wound around the rod. However, heat losses to the mounting are significant and increased input power is required to compensate for that heat loss. Moreover, configuration of the bombardment arrangement is not compatible with a uniform accelerating or extraction field, nor is a cathode material having uniform electron emission used.
IBM TDB Vol. 26, No. 10A, March 1984, teaches dual stages of indirect heating of cathode structures. However, this approach is used to avoid alloying of lanthanum hexaboride of the indirectly heated cathode with the directly heated filament. Such alloying tends to weaken the filament. Accordingly, the lanthanum hexaboride cathode is surrounded with a tantalum or molybdenum heater cylinder having its interior coated with lanthanum hexaboride to protect the filament. No provisions or adaptations are disclosed therein directed to developing a large area cathode or high uniformity of the temperature of the cathode.
Accordingly, it is seen that the current level of skill in the art does not answer a need for a high current cathode having a large area to support high emittance while maintaining uniformity of temperature and electron emission over the large cathode area.